Red and gray are my favorites as well, but now we have bugeyes as a source of MC and general voodoo fuckery, I feel I might try a run with the gold or even the green one.
With the latest update letting you choose between recruiting regular gals or slave soldiers from the start, I think an opportunity to give the gold codex some love has come up. To me, the gold codex seems to (at least partially) be themed around slaves, so, what if the gold codex gave you some research that allowed you to enhance the regular slave soldier? Maybe something simple like armor (early access to doomguy armor or something, even), or even some sort of tech that turned regular soldiers into something better.
Hell, considering we can turn reapers into green reaper things that spit acid, maybe this could be a part of the superhero arc thing, turning slave soldiers into hulks that suck at using guns buy are great at melee.
These are totally biased suggestions, mind you, since due to the new update I decided to run a game using mostly non ubers, while having an elite team of gals to deal with the really scary stuff.